1. Field of the Invention
This invention relates to polymer chemistry, and in particular to the production of expandable polystyrene (EPS) which contain an expanding agent (EA).
2. Discussion of Related Art
EPS is capable of expansion under the effect of heating. Such heating process results in formation of porous material named polystyrene foam (PSF). PSF features a widespread application in different fields of industry. For example, PSF panels are widely used in construction for the purpose of thermal protection and sound insulation. In addition, PSF is used as all-purpose packaging material including solid wrapping and intricate profile packing for transportation of technical instruments and equipment. PSF can also be used as a shock pad under the rails and for similar applications.
Each application requires certain physical and mechanical properties of PSF. The essential physical and mechanical property is the packed density of the expanded product. Plates which are used as shock pads for the railroad and tram rails must have the packed density of a minimum of 35 kg/m3 or even better, that of a minimum of 45 kg/m3. Panels used in construction have packed density of 25 to 35 kg/m3. PSF packing materials are normally very light demonstrating packed density of 12 to 20 kg/m3. Thus, packed density of the polystyrene foam products, which are manufactured of expandable polystyrene, has the highest priority from the customer's point of view.
PSF can be produced by extrusion. In such cases, polystyrene is melted in the extruder. The EA and other additives are loaded into the hot melt and the foam product is extruded. Such an extrusion method for PSF production is known from European Patent Reference EP 0445847 A2. According to this known method, polystyrene is melted in the melting zone of an extruder. Then an EA, which is selected from the group of freons of different compositions, saturated hydrocarbons with the chain length of C2-C3, carbon dioxide or their mixture, is loaded. The polymer melt is mixed with the EA in the melting zone. Following this step, the mixture is cooled down in the cooling zone and the foam product is extruded. The surface finish of the expanded product from this method is controlled by maintaining the certain range of pressure differential between the inlet of the mixing zone and the extruder bore.
Only special grades of polystyrene having narrow molecular-weight distribution and low melt flow index (MFI g/10 minutes) can be used for production of PSF by the extrusion method. Such stringent requirements to polystyrene are due to the specific character of the extrusion based method of expansion. They limit the foam product line substantially and increase their cost. In particular, the method of extrusion essentially does not allow producing light foam products with packed density of less than 30 kg/m3. Another restriction of the extrusion method is its narrow range of products. For instance, it is possible to manufacture products only in form of film, plates, panels with a maximum thickness of 50 cm, and in sections.
If light foam products of another shape are to be manufactured, another EPS is used, which is produced by granulation polymerization of styrene including wetting out of polymer beads and EA in the polymerization step. Such granulation polymerization based method of EPS production is known, such as from one or more of the following Patent References: JP 491141, U.S. Pat. No. 5,240,967, RU 2087486 C1, FR 2725995 A1, U.S. Pat. No. 5,616,413, DE 19548311 A1, DE 19642658 A1, FR 2820427 A1. This method results in a polymer with a comparatively low molecular weight of less than 200000 and with a melt flow index of the melt of 3 and higher. This expandable suspension polystyrene is suitable for production of foam products of any shape having packed density of 25 kg/m3 and less.
However, the production of plates, or other high density products, using expandable suspension polystyrene is unreasonably costly due to the raw material consumption (EPS) and the energy required for its processing. This is unreasonable, because it is not possible to reach the same quality as that of PSF produced by method of extrusion.
Furthermore, the suspension-based method results in large amounts of effluent water to be treated. The beads of EPS produced by the above suspension-based method are featured by wide particle size distribution. However, the production of foam products uses only EPS beads of certain grain size distribution. This fact requires an additional screen sizing of beads with diameters or grain sizes of over 1.5 mm [see for example Chem.-Ing. Techn., 1996, v68, No. 10, p. 1200] as well as recycling of the end beads and powdery residues. This procedure is hindered because of presence of EA.
Another known method, from U.S. Pat. No. 5,000,801, for the production of beads or grains of EPS with narrow particle size distribution includes mixing the EPS and a nucleation agent. The EPS beads are produced by the suspension-based method and contain 5.9 to 7.5% (by weight) of EA. The nucleation agent is provided by a mixture of citric acid and soda, and is taken in the amount of 0.25 to 0.4 g per 1 kilogram of EPS. EPS beads and nucleation agent are mixed in the mixer and then delivered to the extruder. The polymer is melted. The polymer filaments are extruded under the temperature of between 115° C. and 125° C. and the pressure of 1800 psi to 2000 psi, and under conditions which prevent the expansion. These conditions comprise a water bath with a water temperature of approximately 22° C. (15° C. to 30° C.). In this water bath, the extruded filaments are drawn with the rate which is 1.8 times higher than the extrusion rate. The stretched filaments are cut into the polystyrene beads containing EA.
One significant disadvantage of this method is the use of polystyrene with molecular weight (Mw) of less than 200000, or of 90000 to 150000 to be exact, and with the melt flow index of the melt of 4.5 to 5.0. Another disadvantage is that the primary polymer may not contain any additives such as antistatic agents, internal lubricants, which prevent clumping of beads and the like, which limits the application of this method significantly.
The method for production of the expandable polystyrene beads is known from European Patent Reference EP 0668139 A1. According to this method, the polystyrene melt flow and the flow of EA is fed into the mixing zone. The EA is dispersed in the polymer melt by thorough shear mixing in the first static mixer. The mixture prepared as above is held under thorough shear mixing in the second static mixer. This mixture is cooled down under mixing in the third static mixer, to the intermediate temperature and then with after-cooling of the mixture, down to the temperature required for granulation. Polymer filaments are extruded by an extrusion nozzle on their shock cooling and are cut in granulate. According to this method, the polymer melt can be supplied to the static mixer both from the extruder and from the installation for the production of polystyrene by bulk polymerization. Holding the mixture with simultaneous shear mixing enables the diffusion process of the EA in the polymer melt. At the cooling stage, the polymer melt is cooled down to the temperature of approximately 120° C. At that temperature, the pressure falls down to approximately 10 MPa. At the extrusion nozzle, the pressure falls down to 1 MPa. The polymer filaments are extruded into the water bath where a temperature of approximately 10° C. is maintained. This results in the formation of EPS beads with uniform distribution of EA and of equal size.
One disadvantage of this known method is that it is suitable for the subsequent processing of only one type of polymer, such as only polymer with certain narrow molecular-weight distribution, in order to produce EPS.